Project Summary Scattered radiation remains a major problem in CBCT imaging that degrades soft tissue visualization and quantitative accuracy. In image guided radiation therapy (IGRT), poor CBCT image quality due to scatter corrupts the accuracy of patient setup corrections and prevents the use of CBCT in radiation therapy (RT) dose calculations and automated tissue segmentations. The latter issue in particular is a major roadblock to enable patient-specific adaptive radiotherapy, whereby the treatment plan is modified during the course of treatment in response to tumor and normal tissue anatomic changes during treatment. To mitigate the scatter problem, software-based scatter correction methods and hardware-based scatter rejection devices, such as radiographic antiscatter grids (ASGs), have been heavily investigated in the past decade. However, the desired improvement in CT number (Hounsfield Unit) accuracy and low contrast sensitivity has not been achieved. Thus, to overcome the shortcomings of current scatter suppression methods, we will develop a novel scatter rejection device, a two dimensional focused antiscatter grid (2D ASG), dedicated for CBCT systems used in IGRT. The scatter suppression advantage of a 2D array over a radiographic ASG, is well known. However, construction of a 2D ASG with favorable primary x-ray transmission properties has not been achievable using standard manufacturing processes. To address this challenge, we will employ a 3D printing technology, to build the 2D ASG from tungsten. By utilizing 3D printing, septal thickness of the 2D ASG will be significantly reduced and each through-hole will be precisely aligned towards the x-ray focal spot. Furthermore, aluminum or fiber inter-septal spacers as in radiographic ASGs will be eliminated due to the mechanical strength of the tungsten 2D array. We hypothesize that the proposed design will enable significant enhancement in soft tissue structure visualization, and it will significantly reduce CT number calculation inaccuracy due to its high primary transmission levels and efficient scatter suppression capability. To test our hypothesis, we will (1) simulate and build 2D ASG prototypes using 3D printing technology, and characterize their signal transmission properties, (2) evaluate the impact of 2D ASG on CBCT image quality, (3) validate improvements in RT dose calculation accuracy. We will perform the experiments using a benchtop CBCT system due to its flexibility to test various ASG prototypes. The final, optimized 2D ASG prototype will be suitable to install in clinical CBCT systems for further imaging experiments or clinical trials. The outcome of this project can also be translated to other CBCT modalities that have fixed source-detector geometry